1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device and more particularly to a method for fabricating a semiconductor device, wherein an ashing of a resist film formed over a ruthenium film or a ruthenium oxide film can be performed at a high selectivity.
2. Description of the Prior Art
In recent years, semiconductor devices including ruthenium or ruthenium oxide have been actively studied and developed in the art. Among them, a capacitor having electrodes made of ruthenium or ruthenium oxide and a dielectric material such as Pb(ZrxTi1xe2x88x92x)O3, (BaxSr1xe2x88x92x)TiO3, or Ta2O5 has been employed in some Ferroelectric Random Access Memory (FeRAM) or Dynamic Random Access Memory (DRAM).
Ruthenium and ruthenium oxide are capable of being processed into a high-anisotropic form by reactive ion etching using oxygen gas (e.g., U.S. Pat. No. 5,254,217 Maniar et al.) and a gas mixture of oxygen gas and halogen gas (e.g., U.S. Pat. No. 5,624,583 Tokashiki et al.). While it become possible to make the size of a capacitor electrode minimized with above etching method, there is a necessity to conduct an ashing of a photo-sensitive material (i.e., photoresist) formed over the electrode made of ruthenium or ruthenium oxide using oxygen plasma.
FIGS. 1a to 1e are cross sectional views for illustrating substantial steps in an exemplified conventional method for fabricating a semiconductor device having a ruthenium (Ru) film, wherein the step of ashing is performed after the step of contact hole etching.
First, as shown in FIG. 1a, a Ru film 12 and a silicon dioxide (SiO2) layer 14 are formed over a silicon (Si) substrate 11. As shown in FIG. 1b, a photoresist is then applied on the SiO2 layer 14 followed by a patterning step using photolithography process to form a resist pattern 13. As shown in FIG. 1c, subsequently, a contact hole 16 for connecting a wiring to the Ru film 12 is formed through the SiO2 layer 14 by a dry etching process using CF4 or the like. After that, as shown in FIG. 1d, the resist pattern 13 is subjected to a plasma ashing process using O2 gas. During the ashing, the Ru film 12 is also gradually eroded because of the formation of volatile RuO3 or RuO4 as a result of reacting Ru with oxygen plasma 22. Consequently, the complete removal of the resist pattern 13 involves a substantial erosion of the Ru film 12. In some cases, the Ru film 12 under the contact hole 16 may be vanished as shown in FIG. 1e. 
In the described conventional example, the ashing is performed after the contact hole etching. In the actual fabrication of the semiconductor device, there may be cases where such an ashing step in which the Ru film 12 may be exposed to oxygen plasma is performed more than one time. Thus, there is a possibility of vanish away the Ru film 12 from the structure after repeating the ashing step several times even though the entire Ru film 12 cannot be removed by the one ashing step.
For solving such a problem, Yunogami et al. (U.S. Pat. No. 6,326,218) disclose a method for preventing the Ru film 12 from erosion at the time of ashing the resist pattern 13 by forming a platinum (Pt) film 15 on a Ru film 12.
FIGS. 2a to 2e are cross sectional views for illustrating the steps in such a method disclosed in the above document. In this method, as shown in FIG. 2a, at first, the Ru film 12 is deposited over a Si substrate 11. Then, the platinum (Pt) film 15 is deposited on the Ru film 12. After patterning of the Pt film 15 and Ru film 12, a SiO2 layer 14 is deposited on the Pt film 15. Then, as shown in FIG. 2b resist pattern 13 is formed on the SiO2 layer 14. Furthermore, as shown in FIG. 2c, a contact hole 16 is formed through the SiO2 layer 14 by a dry etching process using CF4 or the like. After that, as shown in FIG. 2d, the resist pattern 13 is subjected to a plasma ashing process using oxygen plasma 22. In this case, as the Ru film 12 is covered with the Pt film 15, the Ru film 12 is not eroded at all, while the resist pattern 13 is completely incinerated by the ashing process as shown in FIG. 2e. 
As described above, in the method for fabricating the conventional semiconductor device, the Ru film 12 may be eroded at the time of ashing. For preventing the Ru film 12 from the erosion, there is a necessity to deposit the Pt film 15 on the Ru film 12 to prevent it from erosion or disappearance. In this case, however, the fabrication becomes more complicated because of the additional steps of forming and patterning the Pt film 15 on the Ru film 12. In addition, the cost of fabricating the semiconductor device becomes increased because of costly Pt. Consequently, a sufficient productivity cannot be attained in the conventional method for fabricating a semiconductor device including Ru or Ru oxide. Furthermore, in some case, the formation of Pt film 15 on the Ru film 12 cannot be done as the need for directly forming a dielectric layer on the Ru film 12 comes out.
It is an object of the present invention to provide a method for fabricating a semiconductor device with a high productivity, wherein an ashing of a photo-sensitive material formed over ruthenium or ruthenium oxide can be easily realized at a high selectivity.
In a first aspect of the present invention, a method for fabricating a semiconductor device including at least one of a ruthenium and a ruthenium oxide comprises the step of ashing a photo-sensitive material over the ruthenium or the ruthenium oxide using a gas mixture containing oxygen gas or ozone gas and nitrogen gas, wherein the percentage composition of nitrogen gas is 50% or more.
Here, the ashing step may be performed by heating a substrate over which the ruthenium or the ruthenium oxide is formed at a temperature of 200xc2x0 C. or more.
The ashing step may be performed after etching an interlayer insulation film on the ruthenium or the ruthenium oxide using a photo-sensitive material as a mask. Alternatively, the ashing step may be performed after etching of the ruthenium or the ruthenium oxide using a photo-sensitive material as a mask.
In a second aspect of the present invention, a method for fabricating a semiconductor device including at least one of a ruthenium and a ruthenium oxide, comprising the steps of: forming a film made of the ruthenium or the ruthenium oxide over a substrate; forming an interlayer insulation film on the ruthenium or the ruthenium oxide film; applying a photo-sensitive material on the interlayer insulation film and patterning the applied photo-sensitive material; etching the interlayer insulation film using the patterned photo-sensitive material as a mask; and ashing the patterned photo-sensitive material using an ashing gas provided as a mixture of a gas that contains oxygen gas or ozone gas and a gas that contains nitrogen gas, wherein the percentage composition of nitrogen gas is 50% or more.
Here, a contact hole for exposing the ruthenium film or the ruthenium oxide film may be formed in the step of etching the interlayer insulation film.
According to the above first or second aspect of the invention, there is an advantage of effectively performing the ashing of the photo-sensitive material with a high selectivity and a high ashing rate while preventing a partial disappearance of the ruthenium film or the ruthenium oxide film.
The reason for such an advantage is as follows. In the prior art method, the ruthenium is etched by oxygen plasma because ruthenium is oxidized to form a volatile compound, i.e., RuO3 or RuO4. In this case ruthenium may be eroded or disappeared at the time of performing ashing of the photoresist using oxygen plasma.
On the other hand, according to the present invention the gas for ashing is prepared by mixing O2 gas with a large amount of N2 gas, so that the probability of a collision of oxygen with ruthenium can be decreased. At this time, the ashing rate of photoresist may be slightly decreased with a degree less than the decrease of etching rate of ruthenium. Therefore, the ashing of photoresist with an improved selectivity with respect to ruthenium can be attained.